DisableDebuggerAttachment pretends to be disabled if you change it and hup
The feature from #3313 (moved) does a prctl or ptrace call if it's on during options_act(), and does a log_notice if it's off during options_act.
So I can change it to off, hup, and get the log_notice saying attaching should work. But it doesn't actually do anything to make it work again.
rransom points out quite rightly that our process isn't supposed to be able to do anything to make it work again. So the correct fix is to stop deceiving the user. Perhaps we should make it an invalid transition in options_transition_allowed()?
Also, should we really be calling tor_disable_debugger_attach() on every hup? Seems like the sort of thing that we should have a static variable to remember.
And finally (and related), if the option is off, you get your log_notice on every hup whether anything changed or no.